Gravimeters are instruments for measuring the acceleration of gravity, typically described in units of Gal, wherein Gal=cm/s.sup.2. Gravimeters are frequently used in geological surveys to detect oil reserves and mineral deposits, or measure tidal currents and movements of the earth's crust, among others. Traditional gravimeters rely on measuring the vertical displacement of a mass suspended on a mechanical spring or measuring the free-fall acceleration of a mass from a predetermined height. Torsional pendulums are a variation of the traditional approach often used for basic scientific studies, such as measuring the gravitation constant (G). Torsional pendulums are comprised of a pair of masses suspended from a fiber, whereby the restoring torque of the fiber and the supportable weight are proportionally related to the diameter of the fiber. A ferromagnetic rotor and active magnetic bearings are useful for levitation purposes to alleviate stability and fiber breakage in torsional pendulums, however, the lack of constancy of restoring torque to the fiber has been a persistent problem.
Another approach measures the movement of a superconducting detector mass or sphere levitated by a magnetic field generated by superconducting coils. As shown in FIG. 1, a superconducting gravimeter 10 uses the Meissner effect to suspend a superconducting niobium (or lead) sphere 12, as detector mass, in a magnetic field created by a persistent current in a superconducting coil(s) 14a, 14b, within a vacuum housing 20 containing low-pressure helium gas. The superconductors are Type I, i.e., mainly pure metals featuring perfect diamagnetism up to a critical magnetic field strength, whereupon superconductivity is lost and the magnetization of the superconductor abruptly disappears. Since Type I superconductors are only stable at very low temperatures of about 15K, the superconductor gravimeter 10 is maintained at 4.2K by suspension in liquid helium. The current in the superconducting coil(s) 14a, 14b provides a stable vertical force sufficient to support the weight of the sphere 12, with a vertical force gradient sufficiently weak to allow measurement of changes in the gravitational force. Capacitance sensing plates 16 are arranged symmetrically about the sphere.
Although superconductor gravimeters provide improved stability and sensitivity (about 1 nGal), Type-I superconductivity is lost in very weak magnetic fields, such that the detector masses are severely limited in size and their levitation becomes unstable at low chamber pressures. Type I superconductors are also not stable against rotational movement, and where rotational or vibrational modes are excited within the gravimeter, a time delay must occur to allow these modes to sufficiently decay before measurements are made. Yet another disadvantage of Type-I superconductor gravimeters is their extreme sensitivity to temperature; all low-temperature superconductor gravimeters must be controlled within 5 .mu.K.
The above described gravimeters essentially measure only the vertical component of gravity, and, although measuring other spatial components of gravity is principally possible using the Type I superconductor gravimeter, the necessity of operating in a liquid helium environment makes such a measurement generally impractical.
Recently, low-friction bearings comprised of a permanent magnet levitated over a high temperature superconductor (HTS) have been developed. The levitation is passively stable with moderate stiffness in radial, vertical, and tilt direction, and low resistance to rotational motion. For example, HTS bearings have coefficients of friction as low as 10.sup.-8. Advantageously, HTS materials, such as YBa.sub.2 Cu.sub.3 O.sub.7, exhibit low electrical resistance at or below a critical temperature above the boiling point of nitrogen (77K), and, therefore, HTS applications allow cooling with liquid nitrogen, a significantly more efficient and less expensive refrigerant than liquid helium.
The present invention is a high temperature superconducting (HTS) gravimeter comprising a permanent magnet levitated above or otherwise magnetically held in a spaced relationship with a high temperature superconductor, a cantilever having a mass at one end and connected at its other fixed end to the permanent magnet. Gravity acts on the mass and cantilever, causing measurable rotational acceleration. The HTS gravimeter is highly sensitive (about 1 nGal) and gravitational acceleration is measurable in all spatial directions. In addition, the HTS gravimeter is operable at liquid nitrogen temperatures. The HTS gravimeter may also be miniaturized for specific applications, including oil exploration.
Therefore, in view of the above, a basic object of the present invention is to provide a highly sensitive high temperature superconducting (HTS) gravimeter for measuring the acceleration of gravity in all spatial directions.
Another object of this invention is to provide an HTS gravimeter operable at liquid nitrogen temperatures.
Yet another object of this invention is to provide an HTS gravimeter for measuring the acceleration of gravity in applications having limited space, requiring miniaturization of measurement instruments.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.